1. Field of the Invention
The present invention relates to a method for manufacturing an optical fiber preform to be used in the field of communication and optics.
2. Description of the Related Art
Conventionally, there has been performed a method using an extrusion molding as a method for manufacturing an optical fiber preform. In Japanese Patent Application No. 3-254412, the inventors of the present invention proposed a method for forming a core portion and a clad portion at the same time by use of a vacuum extrusion machine. According to this method, as a base material, there is used a material in which binder and water is added to silica powder for applying plasticity.
In the above method, since a vacuum extrusion machine is used, most of the bubbles in the plastic material are removed when the core and clad portions are formed. However, its deaeration effect is insufficient for the manufacture of the optical fiber preform. Due to this, in order to accelerate deaeration, molding is performed at a relative low speed, and the plastic material is retained in the extrusion machine as long as possible. However, deaeration is not sufficiently performed, and bubbles remain on the obtained preform. There is a disadvantage in the optical fiber, which is obtained by providing a degreasing process and a sintering process to the preform on which bubbles are left, and drawing the obtained optical fiber preform, in that the resulting optical fiber has low mechanical strength. Therefore, as a whole, the yield is worsened in the manufacture of the optical fiber.